1. Field of the Invention
The invention relates to a wheel rim, made of a steel material, for a motor vehicle such as a motor car or lighter and heavy commercial vehicles.
2. Description of Related Art
Wheel rims of this type are usually manufactured from hot-rolled steel or cast or forged from light metal. Steel rims are characterised, in this context, by a favourable manufacturing price and good performance properties. All the same, wheel rims made of light metal were earlier regarded as particularly favourable because of their lower weight with respect to minimising the unsprung weight of the vehicle. It is possible nowadays, however, for wheel rims to be manufactured at a perceptibly more favourable manufacturing price from high strength and yet easily deformable steel sheets, which, because of their lower wall thickness, have an even lighter weight than light metal wheel rims.
According to general linguistic usage, steel rims are usually composed of what is referred to as the “rim” and a “wheel disc”. When in use, the tyre of the wheel, which is formed from the tyre and the wheel rim, sits on the rim, while the wheel disc establishes the connection between the wheel and the respective axle of the vehicle.
The wheel discs and the rims of steel rims are today usually manufactured from hot-rolled high-strength steels, the tensile strength of which lies in the range of 600 MPa. One typical representative of the steels which come into consideration for this purpose is available on the market under the designation “DP-W600”. This contains, as standard, in addition to iron and unavoidable impurities, (in % by weight) up to 0.12% C, up to 0.08% Al, up to 1.5% Mn, up to 0.8% Si, in total up to 1.00% Cr and Mo, up to 0.06% P, up to 0.005% B, up to 0.01% S and in total up to 0.15% Nb and Ti. A dual-phase steel composed in this manner, of high strength and at the same time easily deformable, is also described in DE 199 36 151 A1.
In order to improve the possibilities for the visual presentation of steel rims, and at the same time improve their protection against corrosion, it has been proposed in JP 2001-219701 that the wheel discs of such a steel rim be manufactured from a multi-layer flat material assembled in the form of a sandwich. The first layer of this material consists of a steel sheet, provided with a metallic coating to provide protection against corrosion, and in practical use absorbs the loads imposed on the steel rim. Applied onto the steel sheet layer is a second layer, on which in turn a third layer lies. The second layer can consist of a resin or the like, and, in the sense of a filler material, has the task of preventing the penetration of moisture into the space which may be present between the first and third layer. In this way, the first steel sheet layer is to be provided with additional protection against corrosion. The third layer of the material of the wheel disc is applied, with the known steel rim, on the side which is visible to the observer when fitted to a vehicle. It can be manufactured, for example, from aluminium, stainless steel or another material regarded by the observer as being of particularly high quality. Its function consists essentially of providing the observer with a particularly good visual impression of the steel rim.
Despite the successes achieved in the past with weight reduction, users still wish to reduce the weight of motor vehicle rims still further. This not only contributes to an improvement in the handling characteristics as a result of minimised unsprung weights but also reduces the overall weight of the respective vehicle.
Against this background, the object of the invention was to provide a wheel rim which can be economically manufactured and which, with low weight, can reliably accommodate the forces imposed on such a wheel rim during practical operation.